Large-area transparent conductive coatings including alloyed carbon nanotubes and nanowire composites, and methods of making the same

ABSTRACT

Certain example embodiments of this invention relate to large-area transparent conductive coatings (TCCs) including carbon nanotubes (CNTs) and nanowire composites, and methods of making the same. The σ dc /σ opt  ratio of such thin films may be improved via stable chemical doping and/or alloying of CNT-based films. The doping and/or alloying may be implemented in a large area coating system, e.g., on glass and/or other substrates. In certain example embodiments, a CNT film may be deposited and then doped via chemical functionalization and/or alloyed with silver and/or palladium. Both p-type and n-type dopants may be used in different embodiments of this invention. In certain example embodiments, silver and/or other nanowires may be provided, e.g., to further decrease sheet resistance. Certain example embodiments may provide coatings that approach, meet, or exceed 90% visible transmission and 90 ohms/square target metrics.

This application is a continuation of application Ser. No. 12/659,354,filed Mar. 4, 2010, the entire disclosure of which is herebyincorporated herein by reference in this application.

FIELD OF THE INVENTION

Certain example embodiments of this invention relate to large-areatransparent conductive coatings (TCCs) including carbon nanotubes (CNTs)and nanowire composites, and methods of making the same. Moreparticularly, certain example embodiments of this invention relate totechniques for improving the σ_(dc)/σ_(opt) ratio via stable chemicaldoping and/or alloying of CNT-based films that may be implemented acrosslarge areas on glass and/or other substrates. In certain exampleembodiments, a CNT film may be deposited and then doped via chemicalfunctionalization and/or alloyed with silver and/or palladium. Bothp-type and n-type dopants may be used in different embodiments of thisinvention. In certain example embodiments, silver and/or other nanowiresmay be provided, e.g., to further decrease sheet resistance.

BACKGROUND AND SUMMARY OF EXAMPLE EMBODIMENTS OF THE INVENTION

Carbon nanotubes (CNT) are promising materials for transparentconduction as a result of their exceptional electrical, optical,mechanical, and chemical properties. Ultra thin films based on CNTnetworks above the percolation limit have beneficial attributes such asstiffness and chemical stability that makes it superior to indium tinoxide (ITO) in certain applications. CNT nano-mesh films exhibitflexibility, allowing films to be deposited on pliable substrates proneto acute angles, bending, and deformation, without fracturing thecoating. Modeling work has shown that CNT films may offer potentialadvantages such as, for example, tunable electronic properties throughchemical treatment and enhanced carrier injection owing to the largesurface area and field-enhanced effect at the nanotube tips andsurfaces. It is also recognized that although ITO is an n-typeconductor, such CNT films can be doped p-type and, as such, can haveapplications in, for instance, the anode or injecting hole into OLEDdevices, provided the films are smooth to within 1.5 nm RMS roughness.

Although ITO films still lead CNT films in terms of sheet conductanceand transparency, the above-mentioned advantages together with potentialcost reductions have stimulated significant interest in exploitingcarbon nanotube films as transparent conductive alternative to ITO. Inorder to live up to its expectations, CNT films should display hightransparency coupled with low sheet resistance. The relationship betweentransparency and sheet resistance for thin conducting films iscontrolled by the ratio of dc conductivity and optical conductivity,σ_(dc)/σ_(opt), such that high values of this ratio typically are mostdesirable.

However, to date, viable CNT synthetic methods yield poly-dispersedmixtures of tubes of various chiralities, of which roughly one-third aremetallic with the remainder being semiconducting. The low σ_(dc)/σ_(opt)performance metric of such films is largely related to the largefraction of semiconducting species. These semiconducting tubes, in turn,also give rise to the bundling of the tubes, which tends to increase thejunction resistance of the film network.

The typical value of σ_(opt) for CNT films depends on the density of thefilm. Just above the percolation limit, this value tends to close at1.7×10⁴ S/m at 550 nm, while the dc electrical conductivity to date isin the region of 5×10⁵ S/m. However, industry specifications requirebetter than 90% transmission and less than 90 ohms/square sheetresistance. To achieve these values, one can determine that thenecessary dc conductivity be in excess of 7×10⁵ S/m. Thus, it will beappreciated that there is a need in the art for improving the electronicquality of even the best CNT films so that the σ_(dc)/σ_(opt) ratio, inturn, is improved. This poly-dispersity stems from the unique structureof SWNTs, which also renders their properties highly sensitive to thenanotube diameter.

Certain example embodiments of this invention relate to the depositionof nano-mesh CNT films on glass substrates and, in particular, thedevelopment of coatings with high σ_(dc)/σ_(opt) on thin, low iron oriron free soda lime glass and/or other substrates (e.g., other glasssubstrates such as other soda lime glass and borosilicate glass,plastics, polymers, silicon wafers, etc.). In addition, certain exampleembodiments of this invention relate to (1) finding viable avenues ofhow to improve the σ_(dc)/σ_(opt) a metric via stable chemical dopingand/or alloying of CNT based films, and (2) developing a large areacoating technique suitable for glass, as most work date has focused onflexible plastic substrates. Certain example embodiments also pertain toa model that relates the morphological properties of the film to theσ_(dc)/σ_(opt).

In certain example embodiments of this invention, a method of making acoated article comprising a substrate supporting a carbon nanotube (CNT)inclusive thin film is provided. A CNT-inclusive ink is provided. Theink is applied to the substrate to form an intermediate coating. Amaterial is provided over the intermediate coating to improve adhesionto the substrate. A solution of PdCl₂ is prepared. The intermediatecoating is exposed to the solution of PdCl₂ so that the Pd nucleates atjunctions within the intermediate coating, thereby reducing porosity inthe intermediate coating in forming the CNT-inclusive thin film. Anovercoat or passivation layer is provided over the intermediate coatingfollowing the exposing.

In certain example embodiments of this invention, a method of making acoated article comprising a substrate supporting a carbon nanotube (CNT)inclusive thin film is provided. A CNT-inclusive ink is provided, withthe CNT-inclusive ink comprising double-wall nanotubes. The ink isapplied to the substrate to form an intermediate coating using a slotdie apparatus. The intermediate coating is dried or allowed to dry. Anadhesion-promoting layer is provided over the intermediate coating toimprove adhesion to the substrate. The intermediate coating is dopedwith a salt and/or super acid so as to chemically functionalize theintermediate coating. A solution of PdCl₂ is provided. The intermediatecoating is exposed to the solution of PdCl₂ so that the Pd nucleates atjunctions within the intermediate coating, thereby reducing porosity inthe intermediate coating in forming the CNT-inclusive thin film. Anovercoat or passivation layer is applied over the intermediate coatingfollowing the exposing.

In certain example embodiments of this invention, a method of making acoated article comprising a substrate supporting a carbon nanotube (CNT)inclusive thin film is provided. A CNT-inclusive ink is provided, withthe CNT-inclusive ink comprising double-wall nanotubes. The ink isapplied to the substrate to form an intermediate coating using a slotdie apparatus. The intermediate coating is dried or allowed to dry. Anadhesion-promoting layer is provided over the intermediate coating toimprove adhesion to the substrate. A solution of PdCl₂ is provided. Theintermediate coating is exposed to the solution of PdCl₂ so that the Pdnucleates at junctions within the intermediate coating, thereby reducingporosity in the intermediate coating in forming the CNT-inclusive thinfilm. A silvering solution is provided. The intermediate coating isexposed to the silvering solution to short junctions in the intermediatecoating in forming the CNT inclusive thin film. An overcoat orpassivation layer is applied over the intermediate coating following theexposing.

The features, aspects, advantages, and example embodiments describedherein may be combined to realize yet further embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages may be better and morecompletely understood by reference to the following detailed descriptionof exemplary illustrative embodiments in conjunction with the drawings,of which:

FIG. 1 a shows the raw Raman spectrum of a typical pristine undopedfilm;

FIG. 1 b shows the G and D peaks, and the ratio of their intensities isassociated with the level of perfection of the graphitic lattice;

FIG. 2 a is a scanning electron micrograph (SEM) picture of a typicalCNT film on glass;

FIG. 2 b is a scanning electron micrograph (SEM) picture of a compositePEDOT/PSS embedded into the CNT as the network is approximatelyone-quarter full, in accordance with an example embodiment;

FIG. 3 a shows the temperature dependence of the thermoelectric powermeasured for both as-deposited and H₂SO₄ chemically modified samplesproduced in accordance with an example embodiment;

FIG. 3 b shows high resolution FTIR spectra data, indicating chemicaldoping by the SO₄ group at around 1050-1100 cm⁻¹, in accordance with anexample embodiment;

FIG. 3 c is an XPS graph showing a shift as between undoped CNT filmsand CNT films doped in accordance with example embodiments of thisinvention;

FIG. 4 is a bend diagram showing the density of states (DOS) for a 1.7nm semiconducting double-wall tube;

FIG. 5, which plots Tvis vs. Rs for undoped, doped, and composite dopedCNT thin films produced in accordance with an example embodiment;

FIG. 6 is a flowchart describing an example process for alloying withpalladium and/or silver in accordance with an example embodiment;

FIG. 7 is a chart demonstrating pre- and post-alloying visibletransmission and sheet resistances for a variety of samples produced inaccordance with an example embodiment;

FIG. 8 is a cross-sectional schematic view of a touch screenincorporating CNT-based layers according to certain example embodiments;

FIG. 9 is a flowchart illustrating an example technique for forming aconductive data/bus line in accordance with certain example embodiments;

FIG. 10 is an example cross-sectional view of an OLED incorporating aCNT-based coating in accordance with an example embodiment;

FIG. 11 is a cross-sectional schematic view of a solar photovoltaicdevice incorporating graphene-based layers according to certain exampleembodiments;

FIG. 12 is a flowchart showing an illustrative technique for applyingand chemically functionalizing a CNT-based ink in accordance with anexample embodiment;

FIG. 13 is a flowchart shown an illustrative technique for applying andalloying and/or chemically functionalizing a CNT-based ink in accordancewith an example embodiment; and

FIG. 14 is a transmission electron microscope (TEM) image of silvernanowires produced in accordance with an example embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

While thin films made from random meshed networks of carbon nanotubeshave been successfully deposited on various transparent substrates,further improvements are necessary before they can be used inphotovoltaic devices and other electronic applications such as, forexample, OLEDs. Certain example embodiments, however, relate tosolution-deposited smooth thin films made from chemically altered doublewall nanotubes and composites that have stable sheet resistances below100 ohms/square at visible transmittance levels of above 83.5%. Asdescribed in detail below, the effect of modifying the carbon nanotubescan be verified using thermopower vs. temperature measurements, andchanges in optoelectronics properties of the altered films related toweathering may be studied via using SEM, XPS, IR/Raman and spectraltransmittance measurements. Certain example embodiments also relate toapplications of doped films on glass, namely, capacitive touch sensorelectrodes and functional coatings in a fast defogging device. In bothcases, these films have the potential of being viable alternatives toconventional transparent conductive oxides.

Grown carbon nanotubes' hydrophobic nature coupled with tendency toclump in solution has presented many fabrication challenges that limitthe workability of the material. To date, researchers have utilized themethod of vacuum filtrating aqueous solutions of carbon nanotubes toform thin carbon nanotube mats on filtration paper, commonly termedbucky-paper. However, the highly porous material is brittle and fragiledue to the relatively weak van der Waals forces between tubes. In orderto fully harness the mechanical properties offered by carbon nanotubes,uniform and dense distribution of nanotube connectivity throughout thefilm is desirable. In response to this limitation, certain exampleembodiments involve derivatizing of the CNT into a workable water-basedink compatible to glass and using a vertical slot coating technologythat is both scalable and capable of achieving the electro-optical filmquality at a high throughput.

High quality CNT tubes with length distribution from 5-10 microns wereprepared using the catalytic CVD technique. This process produces amixture of nanotubes, including some individual SWNTs and mostly DWNTsof individual average diameter about 1.4 nm. These nanotubes arechemically robust and can be produced in large volumes. The resultingpurified CNTs are then solubilized and dispersed with the help ofsurfactants into water at low power sonication to form a precursor ink.Coating aids were used for tuning the ink rheology and coatingcapability onto glass substrates. Such coating aids may include, forexample, BTAC, DMF, NPH, and/or the like. This ink may also be coatedonto a variety of rigid or flexible substrates (e.g., glass, plastic,metal, silicon, etc.). CNT thin films on thin soda-lime glass substrateswere deposited using a vertical slot method, which provides manybenefits including, for example, higher line speed capability andgreater uniformity over large areas than spray techniques. Thepre-metered vertical slot heads have been designed to exactingtolerances based on the rheological characteristics of the ink fluid.The fluid rheology design parameter encodes the ratio of viscosityverses shear rate at a specific temperature and is used to design theinternal flow geometry. The body sections may be disassembled and splitapart for cleaning. A slot helps to maintain the fluid at the propertemperature for application, distribute it uniformly to the desiredcoating width, and apply it to the glass substrates. Direct setting ofthe flow rate helps determine the wet thickness of the coated film.These techniques involve a precision liquid delivery system and a slothead for widthwise distribution. Substantially uniform coatings areproduced on glass without ribbing and extremely low defect counts. Thesetechniques may include, for example, an apparatus available from TokyoElectron and/or Shafley techniques.

The slot coating lends itself well to applying multilayer coatings. TheCNT film wet thickness is in the range of several tens of microns and israpidly dried at 70-90 degrees C. so as to produce final CNT filmthicknesses in the range of 5-100 nm. CNT films on glass substrates weresubsequently subjected to a 10 minute 9 M H₂SO₄ acid soak or a gas basedsulphonation process, which reduces the conductivity of the filmsubstantially. In order to enhance the adhesion between nanotube thinfilms and glass substrate as well as stabilize the doped films, a 3-5 nmthick PVP polymer overcoat is applied using a similar slot process so asto encapsulate CNT films. The sulphuric acid treatment surfacefunctionalizes the CNT surface by forming both carboxylic and SOOHgroups. It will be appreciated that other “super-acids” may be used tofunctionalize the film in different example implementations.

In addition to, or in place of, the PVP overcoat, an overcoat orpassivation layer may be applied over the functionalized CNT thin film.Such an overcoat or passivation layer may help protect the film fromwater in the event that the acid leaches away, help protect people whomay come into contact with any acid that has leached away, and/orprotect the underlying film (e.g., from burning away, etc.). Such acoating may be a thin film layer of ZnO, zirconium oxide, silicon oxide,silicon nitride, silicon oxynitride, silicon cardbine, etc. Such acoating also may be a polymer-based layer, a resin (such as epoxy), etc.A UV-blocking coating also may be used for an overcoat/passivationlayer.

In order to further stabilize the CNT coating,poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)(PEDOT:PSS)-PEG composite thin films are slot coated from aqueousdispersion. Polyethylene glycol (PEG) additive in Baytron P500 helpsincrease the conductivity of the PEDOT:PSS. In addition, the PEG hasnumerous ether groups containing oxygen in between the terminal hydroxylgroups. When the PSC, containing free ungrafted PEG additive, is coatedonto the CNT functionalized with carbonyl groups, the hydroxyl groups onthese free ungrafted PEG molecules react with the carboxylic groups onthe CNT walls. This causes the PEG to be grafted onto the H₂SO₄functionalized CNT. PEG-PEDOT:PSS are bonded to the CNT walls throughhydrogen bonding of the ether groups of grafted PEG and terminalhydroxyl groups of the free ungrafted PEG. The higher stability stemsfrom a reduced tendency to take up water from the air, which isattributed to a denser packing of the PEDOT:PSS:PEG/CNT composite. Thesheet resistance and roughness of the films were measured again aftercoating with PSC solution. As a control PSC solution was also coatedonto bare soda-lime glass substrates to evaluate the actual sheetresistance and roughness of the spin-coated film, and the results ofthis testing are provided below.

It will be appreciated that an as-deposited film may be placed in avacuum or oven so as to help dry the coating and/or remove any excesswater. Still further, it will be appreciated that the functionalized CNTthin films may be thermally tempered.

Chemical functionalization also may be performed using more permanent orstable dopants. These techniques may be used in place of, or incombination with, the super acid approach described above. For example,chemical functionalization of CNTs by diazonium salts is possible. Forexample, 4-bromobenzenediazonium tetrafluoroborate (BDF) and/ortriethyloxonium hexachloroantimonate (OA) may be used to dope the CNT.BDF tends to extract electrons from the CNT and release nitrogen. Thereaction is driven by the formation of a stabilized charge transfercomplex and will lead to p-type doping of the CNTs. Also, using OA as aone-electron oxidant leads to a similar doping state. The devices weretreated with either a 5.5 mM solution of BDF in water for 10 min or witha 2.7 mM solution of OA in chlorobenzene for 12 hours. After chemicalmodification, the samples were annealed at 100 degrees C. in air. Bothchemical reactions lead to hole injection into the CNTs andpreferentially affect defects in the sidewalls of the CNTs. Conditionsmay be optimized so that the likelihood of introducing additionalstructural defects is reduced and/or eliminated.

As another example, a polyol method may be used so that a metal saltprecursor (e.g., including bromine and/or iodine) is reduced by apolyol, which is a compound containing multiple hydroxyl groups. Thepolyol used in this synthesis, ethylene glycol, served as both thereducing agent and solvent. 10 mL of ethylene glycol was heated at 150degrees C. for one hour with stirring (260 rpm). This pre-heating wasdone in disposable glass vials placed in an oil bath. 40 μL of a 4 mMCuCl₂.2H₂O/ethylene glycol solution was added, and the solution wasallowed to heat for 15 minutes. 1.5 mL 114 mM PVP/ethylene glycol wasthen added to each vial, followed by 1.5 mL 100 mM AgNO₃/ethyleneglycol. All reagents were delivered by pipette. The reaction was stoppedwhen the solution became gray and wispy, after approximately one hour.The reaction was stopped by submerging the vials in cold water. Theproduct was washed and mixed into the CNT ink. In this and/or otherways, silver nanowires may be mixed into the ink that is then applied tothe substrate. This may be done in place of, or in addition to, formingsilver nanowires on the substrate (e.g., prior to, during, or followingapplication of modified or unmodified CNT-inclusive ink)

The salt may be changed to silver bromide, and the same polyol reductiontechnique as described above may be used. Although the density andstatistical nature of the Ag wires formed were the same as silvernitrate, silver bromide may provide lower sheet resistances compared tothe salts. UV may be used to photo-induce the reduction of silver andoxidize the Br ions to Br, indicating that the bromine is an activedopant for CNT tubes.

It also has been found that the presence of Li ions in the form of LiPONhas the effect of decreasing the sheet resistance of the pure CNT filmsby at least 50%. The LiPON can be sputtered onto the glass prior to CNTfilm deposition using, for example, mayer rod techniques. In a paralleleffort, the LiPON may be embedded in the glass prior to coating of theCNT ink and then activated by heat treatment.

It will be appreciated that the chemical functionalization approachesdescribed above using super acids and salts will result in p-typedoping. As alluded to above, however, CNTs may also accommodate n-typedopants. N-type doping may be accomplished using the same techniques asthose described above, provided that different dopants are used. Forinstance, dopants such as, for example, Au, Al, Ti, and/or other metalsmay be used in connection with the above-described techniques. Organicchemicals including, for example, polyethylene imine (PEI) also may beused. PEI in particular may be dissolved in methanol. The CNT coatingmay be dipped into it to dope by physiand chemsorbtion.

A remote low energy oxygen or ozone plasma treatment also may be appliedto CNT thin films in place of, or in addition to, the above-describedexample techniques. This process essentially creates COOH radicals. Incertain example embodiments, a simple corona discharge (either positiveor negative or pulsed) is used to breakdown air to produce ozone in aclose area under which the film is exposed to the ozone. The tip of thecorona discharge is brought over the coating at a distance from 5-10 cm.The film is then exposed to the ozone. The time of exposure can bevaried from 1 min. to 10 min. A multi-rod system with tips that developthe corona as the glass travels below discharge may be used toaccomplish this process. Other ozonators also may be used in differentembodiments of this invention. This discharge of ozone proximate to theglass helps to functionalize the deposited CNT film by oxidizing thecarbon, thereby producing functional moieties on the surface of thetubes that help improve the conductivity of the tubes, effectivelydoping the film p-type.

Further detail regarding the results of the example super acid approachdescribed above will now be provided in terms of film characterizationand CNT film/glass adhesion.

The level of defects in the tubes may be quantified using Ramanspectroscopy. For example, FIG. 1 a shows the raw Raman spectrum of atypical pristine undoped film. It exhibits the key features of the CNT'sbreathing modes (˜240 cm⁻¹). The singlet and doublet RBM peaks observedconfirm the presence of both SWNT and DWNT respectively. Raman shiftω_(RBM) is related to diameter via the relation ω_(RBM) (cm-1)≈A/d_(t)+Bwhere A=234 and B˜10 which yields a value of 1.01 nm. For the DWNT,using the Δω_(RBM), it can be deduced that the distance between theinner and outer tubes is ˜0.32 nm. FIG. 1 b shows the G and D peaks, andthe ratio of their intensities is associated with the level ofperfection of the graphitic lattice. This ratio is typically in theorder of 15 and, taken together with the RBM modes, confirms thepresence of extremely thin (˜1.6 nm) and high electronic quality tubes.The lowest lines correspond to data for the silicon substrate alone, themiddle lines correspond to data for single-wall tubes, and the top linescorrespond to data for double-wall tubes.

The scanning electron micrograph (SEM) picture in FIG. 2 a is that of atypical CNT film on glass. One can deduce quite accurately the diameterand length statistics of such a nano-mesh film. As can be seen, the filmis a nano-mesh with the tubes in the plane of the glass substrate. Filmmorphology can be characterized by the porosity and the average bundlediameter (a bundle being composed of individual tubes). The SEMphotomicrograph confirms the Raman data and indicate that the individualDWNT have a diameter of about 1.6 nm and the median bundle diameterabout 18 nm. The film morphology is characterized by its porosity (voidcontent which increases with thinner or sparser film) and the averagebundle diameter (which tends to be lower with better ink exfoliation andsonication). Modeling performed by the inventor of the instantapplication has shown that electrical conductivity increases with lowerporosity. The porosity can be inferred from the ratio of the filmdensity (extracted from floatation technique) to individual tubedensity. The porosity is estimated to be on the range of 40-70%. FIG. 2b is a scanning electron micrograph (SEM) picture of a compositePEDOT/PSS embedded into the CNT as the network is approximatelyone-quarter full, in accordance with an example embodiment. Details ofthe model are provided below.

Atomic force microscope (AFM) measurements were performed on the threeclasses of films deposited, namely, undoped, doped and PSC-coated films.The RMS roughness is found to be about ˜9 nm for the thin films,decreasing to about 4 nm for the PSC-coated films.

Spectral transmittance Tvis and reflectance Rvis of the film on glasssubstrates were measured as a function of CNT film thickness rangingfrom 5 nm to 40 nm. Metallic SWNTs of 1.4-1.6 nm diameter, inparticular, appear to be desirable chiralities for general purposetransparent conduction, as their transmittance in the visible spectrumseems to be the highest around 550 nm. The transmittance of the dopedH₂SO₄ functionalized films is systematically always larger (≦1%) thanthe same film in the undoped state. The films were also opticallycharacterized using ellipsometry using an effective medium approximationto deduce the fill-factor (or porosity).

The sheet resistances (R_(s)) of the films were measured usingfour-point probes capable of highly measurement in the 1-100 and100-1000 ohms/square. As an additional check, contactless electricalsheet resistance measurements were performed using a Nagy. Work functionmeasurements using ultraviolet photoemission spectroscopy show awork-function of about 5 eV for the pristine films, increasing by 0.3 eVfor the chemically modified films.

FIG. 3 a shows the temperature dependence of the thermoelectric powermeasured for both as-deposited and H₂SO₄ chemically modified samplesproduced in accordance with an example embodiment. The activation energyof the films can be seen to decrease, providing clear evidence for theshift of the Fermi level and the doping effect of H₂SO₄ on the DWNTs. Apositive sign of the thermopower indicates that holes are major chargecarriers in both pristine and modified CNT films, which contrast withthe n-type character of ITO, thus opening new possible applications forthese films. FIG. 3 b shows high resolution FTIR spectra data,indicating chemical doping by the SO₄ group at around 1050-1100 cm-1.The FTIR is operating in reflectance mode.

FIG. 3 c is an XPS graph showing a shift as between undoped CNT filmsand CNT films doped in accordance with example embodiments of thisinvention. As can be seen in FIG. 3 c, there is shift to lower energiesin the Carbon K-edge by about 0.35 eV. This is evidence the BDF andH₂SO₄ are chemically bonding. It will be appreciated that the dopantsmay be imbued into or provided on the substrate and then coated with theCNT ink in certain example embodiments. For example, glass may be coatedwith a low density zirconia, and the zirconia may be sulphonated withH₂SO₄. The CNT may then be coated on top of the sulphonated ZrO₂ incertain example embodiments. One example advantage of the ZrO₂ is toanchor the H₂SO₄ moieties and still allow the H₂SO₄ to chemically dope.The FIG. 3 c XPS chart involves ZrO₂:H₂SO₄ overcoated with CNT. Theshift in the Carbon 1s core helps prove that it is possible to stabilizethe dopant, even under bombardment of UV. It is noted that the It isbelieved that the K-edge Shift is related to the SOOH and SOOOH species.

In order to measure thin CNT nano-mesh film adhesion to glass,macroscopic and microscopic pull-tests were performed on coatedsubstrates. The epoxy based pull test was performed over a range ofsamples with film thickness varying from 10 nm to 100 nm composite CNTfilms. The lower bound for the adhesion metric was found to be greaterthan 3.4×10⁶ Pa (500 psi), limited only by the strength of the epoxybond used or tensile failure in the glass. The microscopic-basedadhesion test was performed using an AFM tip to measure the surfaceenergy of adhesion, S, of the films. This technique is highlyreproducible and gives a value of S˜0.12 and S˜0.15 J/m², whichcorresponds to an average of 10⁷ Pa. At the other extreme, van der Waalattraction between two ideal surfaces with attractions of approximately1 J/m² results in a calculated adhesion strength of about 10⁸ Pa. Eventhough van der Waal's bonding is usually considered “weak,” this type ofattraction between two surfaces is large compared to typical adhesionstrengths for coatings. By way of comparison, the upper limit foradhesion measurements using a commercially available pull tester is only5 to 7.0×10⁷ Pa which, is limited by the strength of an epoxy bond. Itis interesting to note that these values corroborate well with thecalculated values of 0.2 J/m² based on DFT calculations performed by theinventor of the instant application. To the extent that each inter-CNTcontact shares a high tensile strength, the adhesion between a fewlayers of the nano-mesh CNT film and a substrate like glass will likelyfail either at the interface region or in the substrate. FIG. 4 is abend diagram showing the density of states (DOS) for a 1.7 nmsemiconducting double-wall tube.

On a nanometer scale, the film consists of a porous mesh-like structuremade up of individual tubes and bundles of very large aspect ratio(L/D≧100) oriented substantially parallel to the substrate. Bundling ismost prevalent among the semiconducting tubes, likely initiated by longrange unscreened van der Waal forces, and produces a diameterdistribution. Unlike the optical conductivity, the de conductivity islimited by the tunneling of charge carriers from bundle to bundle, suchthat the overall dc conductivity depends on the number of conductivepaths through the film and by the number of inter-bundle junctions on agiven path and the average junction resistance. Thus, the σ_(dc)/σ_(opt)or Tvis/R_(s) ratio can be optimized by controlling film morphology, aswell as enriching the proportion of metallic to semiconducting film. Theleft shift of the Tvis vs. R_(s) curve in FIG. 5, which plots Tvis vs.R_(s) for undoped, doped, and composite doped CNT thin films produced inaccordance with an example embodiment, can be explained by the doping ofthe semiconducting fraction, which improves conductivity of theindividual tubes in the network because the morphological structureremains the same. The inter-nanotube junction resistance therefore maybe surmised to be either larger or on the same order of magnitude as theindividual semiconducting tube resistance.

Because the transparent conductive SWNT films have thicknesses lowerthan 100 nm, which is considerably shorter than optical wavelengths inthe visible and infrared, the sheet resistance of these films can berelated to their transmittance:T(ω)=1+Zo/2R*[σ _(opt)/σ_(dc)(ω)])⁻²where σ_(opt) is the optical conductivity, which varies as a function oflight frequency ω, σ_(dc) is the direct current conductivity, and Zo isa constant equal to 300 ohms, the impedance of free space, respectively.After sum-averaging this equation to obtain Tvis, a fit of the measuredspectral transmittance data (from 400 nm to 800 nm) vs. R for thetransparent undoped, doped, and doped composite conductive films, themetric σ_(dc)/σ_(opt) may be calculated:

Description σ_(dc)/σ_(opt) Stability Index Undoped DWNT 0.9 0.97 DopedDWNT 5.5 0.93 Doped DWNT-PSC Composite 10 1.02

Thus, a conductivity enhancement of about 6 times is observed in thechemically altered compared to the pristine films. The composite filmhas an even greater enhancement factor, due to the fact thatPEDOT:PSS/PEG composite imbues the porous network and provides aparallel path to current flow in terms of holes flux. The compositefilms also seem to have a higher stability index defined by the ratio ofthe metric initial verses aged after exposure to humidity and UV lightafter 10 days. The best result so far observed on film group dopedDWNT-PSC composite could be explained by a denser network provided bythe composite, thereby reducing (and sometimes even completelypreventing) the loss of any adsorbed —SOOH species.

In place of, or in addition to, the doping techniques described above,CNT thin films may be alloyed or otherwise metalized, e.g., withpalladium and/or silver. FIG. 6 is a flowchart describing an exampleprocess for alloying with palladium and/or silver in accordance with anexample embodiment. A CNT ink base coat is provided in step S61. Thismay be accomplished in certain example embodiments by providing a rodsize of 5 or 10 in. connection with a slot-die process. The coatedarticle is then placed in a PdCl₂ bath in step S63. The PdCl₂ isprovided at a concentration of 0.01-1.0% by weight, more preferably0.03-0.5, still more preferably 0.05-0.1% by weight. This concentrationmay be reached by providing PdCl₂ at 5% concentration by weight and thendiluting to the selected concentration. This solution is then coatedonto already-deposited CNT film. The film has some porosity (typicallyup to about 65% for the thinnest films). The Pd is effectively provided(electrodelessly) in between the pores, which helps to push moreelectrons into the nanotubes, in turn, boosting electrical conductivity,after exposure for 5 sec to 1 min., more preferably 10 sec to 30 sec.

It possible to alloy or metalize silver in addition to, or in place of,the palladium. In this regard, if silver alloying or metalizing isdesirable in step S65, then the coated article is immersed in a silverbath in step S66. The process involved is similar to the oxidationreaction that takes place in the silver mirror test. In this test, analdehyde is treated with Tollens' reagent, which is prepared by adding adrop of sodium hydroxide solution into silver nitrate solution to give aprecipitate of silver(I) oxide. Just enough dilute ammonia solution isadded to re-dissolve the precipitate in aqueous ammonia to produce[Ag(NH₃)₂]⁺ complex. This reagent will convert aldehydes to carboxylicacids without attacking carbon-carbon double-bonds. The name “silvermirror test” arises because this reaction will produce a precipitate ofsilver whose presence can be used to test for the presence of analdehyde. If the aldehyde cannot form an enolate (e.g. benzaldehyde),addition of a strong base induces the Cannizzaro reaction. This reactionresults in disproportionation, producing a mixture of alcohol andcarboxylic acid.

Regardless of whether the CNTs are alloyed or metalized with silver instep S66, a topcoat may be provided, e.g., over the CNT-based film withthe palladium and/or silver alloyed or metalized CNTs. This topcoat maybe another silver or palladium deposition, e.g., in accordance with theabove. That is, if a second topcoat is desirable in step S67, it maythen be provided in step S68. In one or more steps not shown, anencapsulating overcoat or passivation layer as described above may alsobe provided. In certain example embodiments, a thin film, polymer,resin, and/or other layer may be applied, e.g., using the techniquesdescribed above.

It will be appreciated that the palladium and/or silver alloyingtechniques and processing conditions given below are provided by way ofexample. In another example, a starting PdCl₂ solution (5% in 10% HCl)was diluted with DI water to a chosen concentration (0.25% in 0.5% HClor 0.1% in 0.2% HCl). Silver nitrate (0.01 g) was dissolved in DI water(10 mL). 23 mL of 0.1N sodium hydroxide was added to solution dropwisewith stirring to form a cloudy brown silver oxide percipitate. To theprecipitate solution, 5N ammonia was added dropwise until the solutionclears (˜0.4 mL), indicating the formation of Tollen's reagent. Areducer, GMPMA2000 from Valspar, was added to solution dropwise (2-10mL) with stirring until a black dispersion of silver colloid fullyformed. Glass with a CNT coating was prepared and measured in thestandard fashion was cut down (in one example, to 0.25 m×0.25 m) toreduce solution loss. The glass was dipped into a bath of the PdCl₂solution for a predetermined amount of time (10-30 s, although it ispossible submerge for longer periods of time), and then excess solutionwas blown dry. It is noted that larger samples may be rinsed. The glasswas then dipped into the silvering solution for no more than 10 s, afterwhich it was blown dry. The backs of the samples were cleaned withnitric acid to remove any residue, and then the entire sample was rinsedwith NPA and blown dry to remove residue streaks on the sample front. Itwill be appreciated that this process may be run on a wet mirror line toreach high production levels. Thus, one illustrative advantage ofcertain example embodiments is that existing equipment, e.g., a mirrorline, may be used to create nanowires and/or metalize the CNTs. In suchexample implementations, the CNT deposition may be performed using avertical slot, and a mirror line for alloying. In such cases, instead ofmaking a mirror coating, the reaction may be quenched to deposit only Pdand Ag wires.

FIG. 7 is a chart demonstrating pre- and post-alloying visibletransmission and sheet resistances for a variety of samples produced inaccordance with an example embodiment. As can be seen, sheet resistance(provided in ohms/square) drops dramatically, while visible transmissionremains relatively unchanged. This indicates a marked improvement in theσ_(dc)/σ_(opt) ratio can be achieved using the example alloyingtechniques described herein.

As alluded to above, junctions formed between metallic andsemiconducting tubes (or bundles) are intrinsically electricallyblocking contacts and, on average, limit electrical current flow. Oneway to circumvent this issue is to provide a CNT ink that is entirelycomposed of metallic nanotubes, wherein the chirality is controlled sothat it is metallic or semi-metallic. Unfortunately, it is not presentlypossible to provide such an ink at an industrial scale.

Using commercially available ink from Unidym, the inventor of theinstant application has determined that it is possible to mitigate theseand/or other issues by synthesizing solution-deposited composite filmsof silver nanowires and carbon nanotubes. The silver nanowires providethe long-distance charge transport and reduce the number of resistivecarbon nanotube junctions in a given current path. Meanwhile, thesmaller carbon nanotube bundles provide charge collection in the porousareas of silver nanowire meshes and transport charge to the silvernanowires. Films show comparable sheet resistance and transparencies tomeshes of pure silver nanowires. Testing also shows that the silver isprotected from environmental degradation by the CNT mesh.

More particularly, Ag nanowires were synthesized by the reduction of Agnitrate in the presence of poly(vinyl pyrrolidone) (PVP) in ethyleneglycol. The resulting Ag nanowires were 2-5 microns long and had adiameter of 17-80 nm. To fabricate transparent electrodes using nanowiresuspensions, a volume of the nanowire suspension was dropped on a glasssubstrate with 100 nm thick pre-patterned Ag frit contact pads, and itwas allowed to dry in air for 10 min while agitated on a shaker. Theresulting films were random meshes of Ag nanowires without significantbundling of wires that were substantially uniform over the area of thesubstrate.

A series of high resolution TEM as well as SEM photomicrographs havebeen taken to probe the meshed network of CNT and silver nanotubes.Atomic force microscope (AFM) and STM measurements also have been takento investigate the resistive losses in sparse films of carbon nanotubebundles. An AFM lithography technique used by the assignee of theinstant invention allows the current in a device to be restricted to asingle bundle or single junction, thus enabling the EFM (electron fieldmapping) to provide a map of the potential versus distance along thecurrent path. This allows the measurement of resistance drops that occuralong nanotube bundles and at bundle junctions. Preliminary data hasshown that bundle resistances of approximately 5-9 kΩ/μm and junctionresistances of 20-40 kΩ/μm are possible. These initial numbers suggestthat resistances of bundle junctions are less than resistances ofindividual tube junctions found in the literature (˜1 MΩ/μm).

Transparent conductive oxide (TCO) films on glass substrates are used invarious types of touch panels including analog resistive, projectedcapacitive, and surface capacitive touch panels. ITO currently is theworkhorse coating for most of these applications, whether it depositedon PET, polycarbonate, or thin glass substrates. Unfortunately, cost anddifficulty of wet etch processing (especially in applications where theTCO needs to be patterned as in projected capacitive applications)limits the role of ITO. An opportunity exists for CNT-based coatings tosupplement or completely replace ITO, provided Tvis is above 86% atsheet resistance is about 120 ohms/square or lower. CNT-based coatingsmay be particularly advantageous on curved substrates, where a slotcoater can transfer the coating that can then be laser scribed.

The assignee of the instant application has developed a novel fullyintegrated capacitive-based sensor with embedded electronics that canfingerprint localized touch. See, for example, application Ser. No.12/318,912, the entire contents of which are hereby incorporated hereinby reference. Two sets of orthogonal electrode patterns are createdwithin the doped CNT coating on 0.7 mm glass and PET substrates usinglaser ablation. The substrates are then laminated to form an array offringe effect capacitors formed by the patterned CNT electrodes. A smartcard thin flexible substrate contains the ancillary surface mountedelectronics components.

A touch panel display may be a capacitive or resistive touch paneldisplay including ITO or other conductive layers. See, for example, U.S.Pat. Nos. 7,436,393; 7,372,510; 7,215,331; 6,204,897; 6,177,918; and5,650,597, and application Ser. No. 12/292,406, now U.S. Pat. No.8,080,141 the disclosures of which are hereby incorporated herein byreference. The ITO and/or other conductive layers may be replaced insuch touch panels may be replaced with CNT-based layers. For example,FIG. 8 is a cross-sectional schematic view of a touch screenincorporating CNT-based layers according to certain example embodiments.FIG. 8 includes an underlying display 802, which may, in certain exampleembodiments, be an LCD, plasma, or other flat panel display. Anoptically clear adhesive 804 couples the display 802 to a thin glasssheet 806. A deformable PET foil 808 is provided as the top-most layerin the FIG. 8 example embodiment. The PET foil 808 is spaced apart fromthe upper surface of the thin glass substrate 806 by virtue of aplurality of pillar spacers 810 and edge seals 812. First and secondCNT-based layers 814 and 816 may be provided on the surface of the PETfoil 808 closer to the display 802 and to the thin glass substrate 806on the surface facing the PET foil 808, respectively. One or bothCNT-based layers 814 and 816 may be patterned, e.g., by ion beam and/orlaser etching.

A sheet resistance of less than about 500 ohms/square for the CNT-basedlayers is acceptable in embodiments similar to those shown in FIG. 8,and a sheet resistance of less than about 300 ohms/square isadvantageous for the CNT-based layers.

It will be appreciated that the ITO typically found in display 802itself may be replaced with one or more CNT-based layers. For example,when display 802 is an LCD display, CNT-based layers may be provided asa common electrode on the color filter substrate and/or as patternedelectrodes on the so-called TFT substrate. Of course, CNT-based layers,doped or undoped, also may be used in connection with the design andfabrication of the individual TFTs. Similar arrangements also may beprovided in connection with plasma and/or other flat panel displays.

In yet another variant of this technology the CNT electrodes is beprinted on surface 4 of windshield (or between surfaces 2 and 3respectively). The driver electronics can be either capacitively coupledor in direct contact via pins producing a fractal-based electric fieldsensing system based on CNT coatings via a combination of excitation,return, and shield electrodes. See, for example, application Ser. No.12/453,755, now U.S. Pat. No. 8,009,053 the entire contents of which arehereby incorporated herein by reference. This system is capable ofachieving sensing areas of at 1500 mm² and conforms to the windshieldsurface. The system comprises multiple layers of distributed arraycapacitors stacked on top of each other and electrically isolated andshielded from each other. In this compact design, a flip chip lightsensor also may be integrated to monitor both the visible and IRspectrum for both night vision as well as solar radiation load into thevehicle. See, for example, U.S. Pat. No. 7,504,957, the entire contentsof which are hereby incorporated herein by reference. The sensor mayconsume low power (mW) and have high resolution (millimeter), lowlatency (millisecond), high update rate (1 kHz), and high immunity tonoise (>70 dB).

The above-described light sensors and rain sensors can be used inrefrigerator/freezer door applications, as well. A capacitive sensor maybe provided, which may include at least one CNT-based layer. Whenmoisture or condensation is detected, an active solution may selectivelyheat a CNT-based line or layer so as to reduce condensation. See, forexample, application Ser. No. 12/149,640, now U.S. Pat. No. 7,964,821the entire contents of which are hereby incorporated herein byreference. In such active anticondensation applications, a CNT-basedline or layer may be used to replace ITO or other TCO. This may beparticularly advantageous, in that CNT-based lines or layers are betterable to withstand current, e.g., because they do not degrade or oxidizeas rapidly as some TCOs (including, for example, ITO). Example activesolutions are disclosed in, for example, application Ser. No.12/458,790, U.S. Pat. Nos. 7,246,470; 6,268,594; 6,144,017; and5,852,284, and U.S. Publication No. 2006/0059861, the entire contents ofeach of which are hereby incorporated herein by reference.

Defogging and deicing example embodiments were fabricated with aCNT-inclusive film having a sheet resistance of 10 ohms/square. Thisexample film has advantages over both silver coatings and ITO. Forinstance, there was no sign of corrosion after almost 1000 cycles ofdefog. In comparison, at this number of cycles, the ITO releases oxygenand starts to show color change, and pure silver thin films start tocorrode. The high electric field at the tips seems to also behave in a“crisper” or cleaner manner. As much as 10 KW per meter square wasapplied on a 12×12 sample and, at this level, performance was very good.

CNT-based layers also may be used to create conductive data/bus lines,bus bars, antennas, and/or the like. Such structures may be formedon/applied to glass substrates, silicon wafers, etc. Similarly,CNT-based layers may be used to form p-n junctions, rectifiers,transistors, electronics on glass including, for example, solid statevalves, and/or the like. FIG. 9 is a flowchart illustrating an exampletechnique for forming a conductive data/bus line in accordance withcertain example embodiments. In step S901, a CNT-based layer is formedon an appropriate substrate. In an optional step, step S903, aprotective layer may be provided over the CNT-based layer. In step S905,the CNT-based layer is selectively removed or patterned. This removal orpatterning may be accomplished by laser etching. In such cases, the needfor a protective layer may be reduced, provided that the resolution ofthe laser is fine enough. Alternatively or in addition, etching may beperformed via exposure to an ion beam/plasma treatment. Also, H* may beused, e.g., in connection with a hot filament. When an ion beam/plasmatreatment is used for etching, a protective layer may be desirable. Forexample, a photoresist material may be used to protect the CNT areas ofinterest. Such a photoresist may be applied, e.g., by spin coating orthe like in step S903. In such cases, in another optional step, S907,the optional protective layer is removed. Exposure to UV radiation maybe used with appropriate photoresists, for example.

CNT-based layers also may be used in photovoltaic devices, e.g., insemiconductor and/or absorber layers, provided the sheet resistancethereof can be provided at an appropriate level. CNT-based layers may beparticularly advantageous in such cases, as they can be doped p-type orn-type, as explained above.

As indicated above, CNT-based coatings also may be used in connectionwith OLED displays. A typical OLED comprises two organic layers—namely,electron and hole transport layers—that are embedded between twoelectrodes. The top electrode typically is a metallic mirror with highreflectivity. The bottom electrode typically is a transparent conductivelayer supported by a glass substrate. The top electrode generally is thecathode, and the bottom electrode generally is the anode. ITO often isused for the anode. When a voltage is applied to the electrodes, thecharges start moving in the device under the influence of the electricfield. Electrons leave the cathode, and holes move from the anode inopposite direction. The recombination of these charges leads to thecreation of photons with frequencies given by the energy gap (E=hν)between the LUMO and HOMO levels of the emitting molecules, meaning thatthe electrical power applied to the electrodes is transformed intolight. Different materials and/or dopants may be used to generatedifferent colors, with the colors being combinable to achieve yetadditional colors. CNT-based films may be used to replace the ITO thattypically is present in the anode. CNT-based films also may be used inconnection with the hold-transporting layer.

FIG. 10 is an example cross-sectional view of an OLED incorporating aCNT-based coating in accordance with an example embodiment. The glasssubstrate 1002 may support a transparent anode layer 1004, which may bea CNT-based layer. The hole transmitting layer 1006 also may be aCNT-based layer, provided that it is doped with the proper dopants.Conventional electron transporting and emitting and cathode layers 1008and 1010 also may be provided. For additional information concerningOLED device, see, for example, U.S. Pat. Nos. 7,663,311; 7,663,312;7,662,663; 7,659,661; 7,629,741; and 7,601,436, the entire contents ofeach of which are hereby incorporated herein by reference.

In certain example embodiments, CNT-based films produced in accordancewith the above methods may be used in connection with low-emissivityapplications. For instance, CNT-based films may be provided inmonolithic and insulating glass (IG) windows. These CNT-based films areheat treatable such that the substrates that support them may beannealed or thermally tempered with the films thereon. Because CNT-basedfilms are survivable, they may be provided on any surface of suchwindows. Of course, it will be appreciated that encapsulating them withan overcoat or passivation layer also may help ensure survivability andexposure to the environment.

Another example electronic device that may make use of one or moreCNT-based layers is a solar photovoltaic device. Such example devicesmay include front electrodes or back electrodes. In such devices, theCNT-based layers may simply replace the ITO typically used therein.Photovoltaic devices are disclosed in, for example, U.S. Pat. Nos.6,784,361, 6,288,325, 6,613,603 and 6,123,824; U.S. Publication Nos.2008/0169021; 2009/0032098; 2008/0308147; and 2009/0020157; andapplication Ser. Nos. 12/285,374, 12/285,890, now U.S. Pat. No.8,022,291 and 12/457,006, now U.S. Pat. No. 8,445,373 the disclosures ofwhich are hereby incorporated herein by reference. A photovoltaic devicealso is disclosed in “Highly Absorbing, Flexible Solar Cells WithSilicon Wire Arrays Created,” ScienceDaily, Feb. 17, 2010, the entirecontents of which is hereby incorporated herein by reference, andCNT-based layers may be used in such a device.

Alternatively, or in addition, doped CNT-based layers may be includedtherein so as to match with adjacent semiconductor layers. For instance,FIG. 11 is a cross-sectional schematic view of a solar photovoltaicdevice incorporating CNT-based layers according to certain exampleembodiments. In the FIG. 11 example embodiment, a glass substrate 1102is provided. For example and without limitation, the glass substrate1102 may be of any of the glasses described in any of U.S. patentapplication Ser. No. 11/049,292 now U.S. Pat. No. 7,700,869 and/or Ser.No. 11/122,218, now U.S. Pat. No. 7,700,870 the disclosures of which arehereby incorporated herein by reference. The glass substrate optionallymay be nano-textured, e.g. to increase the efficiency of the solar cell.An anti-reflective (AR) coating 1104 may be provided on an exteriorsurface of the glass substrate 1102, e.g., to increase transmission. Theanti-reflective coating 1104 may be a single-layer anti-reflective(SLAR) coating (e.g., a silicon oxide anti-reflective coating) or amulti-layer anti-reflective (MLAR) coating. Such AR coatings may beprovided using any suitable technique.

One or more absorbing layers 1106 may be provided on the glass substrate1102 opposite the AR coating 1104, e.g., in the case of a back electrodedevice such as that shown in the FIG. 11 example embodiment. Theabsorbing layers 1106 may be sandwiched between first and secondsemi-conductors. In the FIG. 11 example embodiment, absorbing layers1106 are sandwiched between n-type semiconductor layer 1108 (closer tothe glass substrate 1102) and p-type semiconductor 1110 (farther fromthe glass substrate 1102). A back contact 1112 (e.g., of aluminum orother suitable material) also may be provided. Rather than providing ITOor other conductive material(s) between the semiconductor 1108 and theglass substrate 1102 and/or between the semiconductor 1110 and the backcontact 1112, first and second CNT-based layers 1114 and 1116 may beprovided. The CNT-based layers 1114 and 1116 may be doped so as to matchthe adjacent semiconductor layers 1108 and 1110, respectively. Thus, inthe FIG. 11 example embodiment, CNT-based layer 1114 may be doped withn-type dopants and CNT-based layer 1116 may be doped with p-typedopants.

Because it sometimes is difficult to directly texture CNT-based layers,an optional layer 1118 may be provided between the glass substrate 1102and the first CNT-based layer 1114. However, because CNT-based films areflexible, it generally will conform to the surface on which it isplaced. Accordingly, it is possible to texture the optional layer 1118so that the texture of that layer may be “transferred” or otherwisereflected in the generally conformal CNT-based layer 1114. In thisregard, the optional textured layer 1118 may comprise zinc-doped tinoxide (ZTO). It is noted that one or both of semiconductors 1108 and1110 may be replaced with polymeric conductive materials in certainexample embodiments.

Because CNT is highly transparent in the near and mid-IR ranges impliesthat the most penetrating long wavelength radiation may penetrate andgenerate carriers deep into the i-layer of both single and tandemjunction solar cells. This implies that the need to texture backcontacts may not be needed with CNT-based layers, as the efficiency willalready be increased by as much as several percentage points.

Screen-printing, evaporation, and sintering technologies and CdCl₂treatment at high temperatures are currently used in CdS/CdTe solar cellheterojunctions. These cells have high fill factors (FF>0.8). However,series resistance Rs is an efficiency limiting artifact. In Rs, there isa distributed part from sheet resistance of the CdS layer and a discretecomponent associated with the CdTe and graphite based contact on top ofit. The use of one or more CNT-based layers may help reduce bothcontributions to Rs, while preserving good heterojunction properties. Byincluding CNT-based layers in such a solar structure for both front andback contact arrangements, a substantial efficiency boost may beachieved.

It will be appreciated that certain example embodiments may involvesingle-junction solar cells, whereas certain example embodiments mayinvolve tandem solar cells. Certain example embodiments may be CdS,CdTe, CIS/CIGS, a-Si, and/or other types of solar cells.

Certain example embodiments that incorporate doped CNT with Pd andSilver nanowires are able to reach a sheet resistance of 10 ohms/square,on average, with a variance of about 30%. This example coating hasimmediate potential applications, in for example, solar application(e.g., as a TCC). Surface roughness RMS is about 10 nm but, as indicatedelsewhere, the coating may be planarized in any number of ways. Anotherpotential application for this low sheet resistance coating involvessuper capacitors, e.g., for charge storage. Of course, because the inkmay be printed on a wide variety of substrates (e.g., glass, plastics,polymers, silicon wafers, etc.) that may be flat or curved, maydifferent applications also are possible. Indeed, a CNT-based coatingmay be used as a potential antibacterial coating, especially whendisposed in connection with a ZnO layer (or ZnO doping of the ink orsubstrate). Such potential antibacterial behavior may be advantageous inconnection with the refrigerator/freezer door and/or other applicationsdescribed herein.

As indicated elsewhere, CNT-based coatings are suitable to coat curvedsurfaces such as, for example, vehicle windshields. The material tendsnot to thin out in the region where the bend is the greatest. Inaddition, a pattern may be screen printed from ink, e.g., to replacesilver frits. One example where this is possible is for antenna busbars, defogging/deicing applications, and/or the like.

Certain example embodiments also may be used in connection withelectrochromic applications. See, for example, U.S. Pat. Nos. 7,547,658;7,545,551; 7,525,714; 7,511,872; 7,450,294; 7,411,716; 7,375,871; and7,190,506, as well as Application Ser. No. 61/237,580, the entirecontents of each of which are hereby incorporated herein by reference.CNT-based films may replace ITO, as ITO tends to degrade over timeand/or otherwise not perform as well as CNT-based films may.

Details regarding the model will now be provided. The model relies onthe inventor's recognition that the σ_(dc)/σ_(opt) ratio can beoptimized by understanding and controlling film morphology. Moreparticularly, in view of the above, it will be appreciated that theperformance of the CNT-based coating relates' to the network, with thenetwork relating to the mean size of the bundles <D>, mean length ofbundles <L>, fill-factor φ, interconnect density ni, and quality ofindividual nanotube, G/D ratio, and length of NT. Given theserecognitions, the inventor of the instant application derived aphenomenological model that described current results and enablespredictions to be made on the network by studying experimental data. Itis assumed that the thickness is such that the system is above thepercolation threshold in all studied films.

A characteristic length scale or gauge is defined over which theelectrical properties are probed. The scale Lc can then be viewed as theaverage distance between junction. If probed at length scale Lp<Lc, theindividual single or bundle NT conductivities dominate the networkelectrical properties. At the other extreme of Lp>Lc, the length scalespans several junctions. The higher the junction density, the moreparallel paths options exist, thus attenuating the limiting factor thatis the mean junction resistance accordingly the electricalcharacteristics. However, this simple picture is valid if and only ifthe electrical conductivity of the tubes is identical. Thus, filmconductivity is modulated by the individual tube conductivity σ_(NT)which depends on tube chirality, graphitization, dopant level, andlength of tube.

It is then possible to write σ_(f)=f(σ_(NT))*n_(j) as the generalequation on large scale and over scale of L<<Lp σ→σ_(NT).

It is then possible to write n_(j)=n_(b)*<c> where n_(b) is the densityof NT bundles, which is given by:N _(b)=4FF/(π<d ² >*<L>)where L is mean bundle or tube length typically a couple of microns,<d²> is the mean square diameter of the tube bundles, which can bebetween 2 to 20 nm depending on the degree of tube exfoliation. FF isthe film fill-factor, which is equal to ρ_(f)/ρ_(NT) and can beestimated by either the flotation technique or by absorption coefficient{acute over (α)} of the film.

<c> is the mean half-number of junctions formed per tube and may beestimated by the using the following assumptions and reasoning similarto Onsager to deduce (c).

-   -   Mean field approximation where the number density of the        nanotube is average number density.    -   Mean bundle D/L<<1.    -   Contacts are uncorrelated (totally random).

Consider an assembly of (average number density <ρ>) randomly orientedlong rods or strings with large aspect ratio, given a test particle Pand a neighbor N, and with their centers joined by a vector r. In freespace, alone N could adopt any orientation. However, in the presence ofparticle P, a fraction, f_(ex)(r_(b)), of the possible orientations arepossible. This excluded fraction is also the probability that N, withits center fixed at r_(b), will contact P when given a randomorientation. Under assumption, we can then write (1):<c>=½f(r,p)ρ_(n) dr=½<ρ_(n) >∫f(r)dr=½<ρ_(n) ><V _(ex)>  (i)where <ρ_(n)> is the average nanotube (bundle) number density and V_(ex)is the average excluded volume for a distribution of nanotube bundleswhich under the extra assumption that the tube are soft give. Theexcluded volume of a soft-core interpenetrable cylinder is:<V _(ex) >=π<L><D ²>+2<L ² ><D><sin θ>  (ii)

Given that the average volume of each bundle is:<Vp>=π/4{<D ² ><L>}  (iii)from (i) to (iii), an expression for the mean number of contacts perbundle using the fill factor φ of a given mesh will be:<c>=½φ/V _(p) V _(ex)  (iv)

Therefore, the mean junction density n_(j) is given by:n _(j)=½(φ/V _(p))² V _(ex)  (v)

It can be shown from the above equations that can be approximated by:

$\begin{matrix}\begin{matrix}{n_{j} = {{4\varphi^{2}} < D > {/\left( {< D^{2} >} \right)^{2}} < {L\;}^{2} > {/{< L >^{2} < {\sin\;\theta} >}}}} \\{= {{4\varphi^{2}} < D > {/{\left( {< D^{2} >} \right)^{2}\left\lbrack {{{{var}(L)}/} < L >^{2}{+ 1}} \right\rbrack}} < {\sin\;\theta} >}}\end{matrix} & ({vi})\end{matrix}$

Thus, the conductivity of the film depends on the ratio of the meansquare of length of the tubes to the square of the mean. This ratio isbasically the variance of the length distribution to square of the mean.The above analysis also emphasizes the importance of taking the bundlelength and diameter statistical distribution into account whendepositing film networks in potential applications.

In the scale, Lp>>Lc where individual tube or bundle resistancedominates is much less than junction resistance, the film sheetresistance R can be expressed as:R=Rj/(nj*t)  (vii)σ_(f)=k nj/Rj which then leads to the expression for sheet resistance ofthe film of thickness t as a function of the transmission under theregime where film thickness such that {acute over (α)}t<1, thenT′=T/(1−R)=exp(−{acute over (α)}t)=1−{acute over (α)}t.

α is proportional by an effective medium approximation to the fillfactor φ, we then combine all constants into a new constant k″. Bycombining the above equations (under the assumption that the variance inD is very small (such is indeed the case here):R=(πD ³/4kφ ² t)=πD ³ Rj/(4k″φ){var(L)<L²>)+1}<sin θ>*1/(1−T′)which can be written as T′=1−A/R. The bigger fill factor helps explainthe curve as a function of varying the actual density of film. The fillfactor φ is related to porosity by the factor φ=1−P.

Encoded in A are the factors that control the nature of the curve T vs.R The latter analysis can help us understand how the curve is shifted tothe left when doping takes place (e.g., in FIG. 5). All parameters suchas L and D are frozen as well, as fill factor. Rj is affected sincedoping of the semiconducting tubes help decrease the junctionresistance. We surmise that at some point the effect of doping willsaturate the number density of junction is fixed and the dopingefficiency will saturate.

If variance is zero and all tubes were identical with length, thedependence on length would be less noticeable. A is then equal to πD³Rj/(4 k″φ). However that is not the case in practice as evidenced by ourcharacterization of network morphology and statistics of the inkindividual CNT's.

At this juncture, the length dependence of the NT conductivity should betaken into account. This stems from the very large mean free path of thecharge carriers, which is typically around 1 μm for SWCNTs. We surmisefrom calculations based on density functional analysis that for the DWNTthis threshold length is greater than 1 um say (1 to 10 um). Making anindividual DWCNT shorter than 1 μm does not increase its overallresistance any further. Therefore, the conductivity sharply drops atshort lengths of a few nanometers. DWCNTs exceeding 1 μm have an almostone order of magnitude better resistivity than copper, and a SWCNT oflength 100 nm still outpaces the resistivity of W. For DWCNT, the meanfree path is calculated to be higher than 1 um, typically at around 5um. The above-mentioned fact allows a first order approximation ofindividual tube conductivity to be written as a function of tube lengthas Taylor expansion:σ_(NT)=σ_(NTo) +∂σ/∂L*<L>  (viii)

We now take into account the effect of equation (viii) and now R depends(in the limit that variance in L and D are zero) essentially on theindividual tube electrical conductance divided by the number of tubes inparallel in the space of Lp^3. R can be written as the reciprocal of thelength of the tubes. A is now equal to πD³ Rj/(4 k″φ)*1/σ_(NT). Ittherefore is shown how the length of the tube and the prime conductivityσ_(NTo) of the tube also modulate the conductivity of network especiallyon the scale L<Lp. Because as mentioned before the doping effectsaturate, one may envisage using the fact the films have a certaindegree of porosity to nucleate nano-metallic particles whose function isto provide a parallel path for carriers to tunnel form tube to tube at ajunction. Finally, the effect of the juxtaposition of the tubes istreated, and encoded in the factor <sin θ>. This factor is influenced bythe orientation of the tube. To compute the average, the integral of theprobability density function is taken in angle orientation P_(θ)*sin θ.Because the output is a cosine function, this factor tends to amplifythe effect of length if the tubes have some preferred orientation alongthe conductive channel. For a uniform angular distribution we would notexpect any preference in conduction or anisotropy.

This model indicates that the type of tube (metallic or semiconducting)matters when considering the σ_(dc)/σ_(opt) ratio. One partial solutiontherefore is to dope the CNT-based film. The model further indicatesthat doping eventually stops working because the junction resistanceultimately will dominate. This problem may be overcome by alloying ormetallizing, or chemically functionalizing with PEDOT or the like, toshort out those junctions. Finally, the model shows that CNT-based filmsthat the following characteristics are desirable: smaller diametertubes, longer tubes, larger variance in length, and smaller variance indiameter.

FIGS. 12 and 13 briefly review certain of the example techniquesdescribed herein. More particularly, FIG. 12 is a flowchart showing anillustrative technique for applying and chemically functionalizing aCNT-based ink in accordance with an example embodiment. A CNT-inclusiveink is provided in step S1201. The CNT-inclusive ink may comprise orconsist essentially of double-wall nanotubes, e.g., with an averagediameter of approximately 1.7 nm. In step S1203, rheological propertiesof the CNT-inclusive ink may be adjusted, e.g., by adding surfactantsand/or coating aids to the ink so that any semiconducting CNTs locatedwithin the ink are less likely to clump or clot together. In otherwords, the CNT-inclusive ink may be made more water-like. In certainexample embodiments, the ink may be water soluble. Organic and/orinorganic additives and/or solvents may not be necessary in differentexample embodiments of this invention. It will be appreciated that theink may be made to simply dissolve in DI water in certain exampleembodiments, although alcohol may be added in certain exampleembodiments (e.g., to make the water evaporate faster). Optionally, in astep not shown, Ag nanowires may be incorporated into the ink. The inkhaving the adjusted rheological properties may be applied to thesubstrate to form an intermediate coating in step S1205. A slot dieapparatus may be used to accomplish this application. The intermediatecoating is dried or allowed to dry in step S1207. A material (e.g., anovercoat or passivation material) is provided over the intermediatecoating to improve adhesion to the substrate in step S1209. Thismaterial may comprise, for example, PVP, PEDOT:PSS, a PEDOT:PSS-PEGcomposite, zirconia, a silicon inclusive thin film, a polymer or resin,etc. In step S1211, the intermediate coating is doped using a saltand/or super acid so as to chemically functionalize the intermediatecoating in forming the CNT-inclusive thin film. In certain exampleembodiments, the doping may be performed at substantially the same timeas the PVP is provided. In certain example embodiments, the super acidis H₂SO₄, and in certain example embodiments, the salt is a diazoniumsalt (e.g., BDF, OA, or the like). The tubes may be doped p-type orn-type. In step S1213, the film may be substantially planarized, e.g.,using material provided over the intermediate coating or a separateconductive or non-conductive (but perhaps thin) layer. Optionally,oxygen or ozone may be discharged proximate to the substrate tofunctionalize the intermediate coating and/or the CNT-inclusive film byoxidizing carbon located therein. Optionally, in one or more steps notshown, silver nanowires may be synthesized by reducing silver nitrate inthe presence of ethylene glycol (and/or PVP). In certain exampleembodiments, the silver nanowires may be 2-5 microns long and 17-80 nmin diameter. A suspension of synthesized silver nanowires may be droppedon the glass substrate prior to application of the CNT-based ink. Inthis regard, FIG. 14 is a transmission electron microscope (TEM) imageof silver nanowires produced in accordance with an example embodiment.

FIG. 13 is a flowchart shown an illustrative technique for applying andalloying and/or chemically functionalizing a CNT-based ink in accordancewith an example embodiment. A CNT-inclusive ink is provided in stepS1301. The CNT-inclusive ink may comprising or consist essentially ofdouble-wall nanotubes. In step S1303, the adjusting rheologicalproperties of the CNT-inclusive ink may be adjusted, e.g., by addingsurfactants to the ink so that any semiconducting CNTs located withinthe ink are less likely to clump together and/or so that the ink becomesmore water-like. The ink is applied to the substrate to form anintermediate coating (e.g., using a slot die apparatus) in step S1305.The intermediate coating is then dried or allowed to dry in step S1307.In step S1309, a material (e.g., PVP) is provided over the intermediatecoating to improve adhesion to the substrate. Optionally, in step S1311,the intermediate coating is doped so as to chemically functionalize theintermediate coating in forming the CNT-inclusive thin film. Exampledoping techniques are described in detail above. A solution of PdCl₂ isprovided, and the intermediate coating is exposed to the solution ofPdCl₂ in step S1313. The Pd nucleates at junctions within theintermediate coating, thereby reducing porosity in the intermediatecoating in forming the CNT-inclusive thin film. This, in turn, reducessheet resistance while visible transmission remains relativelyunchanged. In step S1315, a silvering solution is provided, and theintermediate coating is exposed to the silvering solution, e.g., toshort junctions in the intermediate coating. The intermediate coatingmay be exposed to the silvering solution after exposing the intermediatecoating to the solution of PdCl₂. The silvering solution may be preparedby dissolving silver nitrate in deionized water. An overcoat orpassivation layer (e.g., comprising PEDOT:PSS, zirconia, a silicon-basedthin film, a polymer, and/or a resins) is provided over the intermediatecoating following the exposing in step S1317. In step S1319, theCNT-inclusive film may be substantially planarized to reduce surfaceroughness. This planarizing may be performed via the overcoat orpassivation layer, or via deposition of an additional layer.

As used herein, the terms “on,” “supported by,” and the like should notbe interpreted to mean that two elements are directly adjacent to oneanother unless explicitly stated. In other words, a first layer may besaid to be “on” or “supported by” a second layer, even if there are oneor more layers therebetween.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of making a coated article comprising asubstrate supporting a carbon nanotube (CNT) inclusive thin film, themethod comprising: providing a CNT-inclusive ink; applying the ink tothe substrate to form an intermediate coating; providing a material overthe intermediate coating; preparing a solution comprising PdCl₂; andexposing the intermediate coating to the solution comprising PdCl₂ sothat Pd nucleates at junctions within the intermediate coating, therebyreducing porosity in the intermediate coating in forming theCNT-inclusive thin film.
 2. The method of claim 1, further comprisingadjusting rheological properties of the CNT-inclusive ink by addingsurfactants to the ink so that any semiconducting CNTs located withinthe ink are less likely to clump together.
 3. The method of claim 2,further comprising using a slot die apparatus to apply the ink havingthe adjusted rheological properties to the substrate.
 4. The method ofclaim 1, further comprising drying the intermediate coating or allowingthe intermediate coating to dry.
 5. The method of claim 1, furthercomprising providing PVP over the intermediate coating.
 6. The method ofclaim 1, further comprising doping the intermediate coating so as tochemically functionalize the intermediate coating in forming theCNT-inclusive thin film.
 7. The method of claim 1, wherein reducingporosity in the coating reduces sheet resistance.
 8. The method of claim7, wherein the sheet resistance is reduced while visible transmissionremains relatively unchanged.
 9. The method of claim 1, furthercomprising substantially planarizing the CNT-inclusive film to reducesurface roughness.
 10. The method of claim 1, further comprisingproviding a silvering solution; and exposing the intermediate coating tothe silvering solution to short junctions in the intermediate coating.11. The method of claim 10, wherein exposing of the intermediate coatingto the silvering solution is performed after exposing the intermediatecoating to the solution comprising PdCl₂.
 12. The method of claim 1,further comprising preparing a silvering solution by dissolving silvernitrate in deionized water; and exposing the intermediate coating to thesilvering solution to short junctions in the intermediate coating. 13.The method of claim 12, wherein exposing of the intermediate coating tothe silvering solution is performed after exposing the intermediatecoating to the solution comprising PdCl₂.
 14. A method of making acoated article comprising a substrate supporting a carbon nanotube (CNT)inclusive thin film, the method comprising: providing a CNT-inclusiveink, the CNT-inclusive ink comprising double-wall nanotubes; applyingthe ink to the substrate to form an intermediate coating; drying theintermediate coating or allowing the intermediate coating to dry;providing an adhesion-promoting layer over the intermediate coating;doping the intermediate coating with a salt and/or super acid so as tochemically functionalize the intermediate coating; providing a solutioncomprising Pd; exposing the intermediate coating to the solutioncomprising Pd so that Pd nucleates at junctions within the intermediatecoating, thereby reducing porosity in the intermediate coating informing the CNT-inclusive thin film; and applying an overcoat orpassivation layer over the intermediate coating following the exposing.15. The method of claim 14, further comprising substantially planarizingthe CNT-inclusive film.
 16. The method of claim 14, further comprisingproviding a silvering solution; and exposing the intermediate coating tothe silvering solution to short junctions in the intermediate coating.